It was hugely instructive to hear two statements made by Arsene Wenger after his side’s 3-1 win against Stoke City at the weekend.

The first was when he confirmed Shkodran Mustafi had suffered a “bad” hamstring strain. Before the Frenchman was pressed into revealing exactly how long the defender might be absent, this correspondent had visions of a Michael Owen-style series of hamstring problems that could be measured in months or even seasons.

So when Wenger announced that “bad” meant a minimum of 21 days, the feeling was it could have been far, far worse.

But given Mustafi’s huge presence at the heart of the defence – in terms of establishing a viable and effective pairing with Laurent Koscielny, and his considerable vocal presence in organising and instructing not only the back line but his colleagues further forward – the absence of the German for any period of time was seen as bad news.

And so it proved in the very first match Mustafi missed – the dismal 2-1 defeat at Goodison Park to an Everton side who hadn’t previously won in 11 games.

For when Toffees full-back Seamus Coleman drifted into the box, unmarked by either Koscielny or Mustafi’s replacement Gabriel, to send a glancing header into the net moments before half-time, it was clear both defenders had erred.

Just as against Stoke, a goal just before the break can change the whole momentum and shape of a game. Theo Walcott’s strike altered the match on Saturday as the Potters lost heart, and so it was on Tuesday for Arsenal as well.

It was dispiriting to see Mesut Ozil walking away from a ball fired into the box with four minutes left, leaving Ashley Williams to win the game with a textbook header. It was clear again that Mustafi’s presence was missing – surely he would have been organising the troops, telling them who to mark and where to stand.

Ozil, who again had a quiet game when faced with a passionate team in a hostile environment, would surely have been more switched on if he had been faced with a German colleague bawling instructions at him.

As it was, Williams’ late goal sealed an unlikely win, based on the opening half-hour of this telling match.

Which brings us to the second revealing insight from Wenger after the Stoke game – he said it was easy to play with less pressure, and harder when expectations were high.

With Arsenal looking to return to the summit of English football on Tuesday, the first half-hour appeared to be a procession back to the top.

Goodison was strangely silent, Everton were without belief and cohesion, passion and aggression.

They were left resorting to hit-and- hope long balls up to Romelu Lukaku until a series of challenges from Ross Barkley, James McCarthy and Aaron Lennon, would you believe, completely changed the atmosphere by rousing the crowd. The home side then began to unnerve Arsenal by going – in manager Ronald Koeman’s familiar refrain – “face to face” with Wenger’s men.

The visitors, who had played with such belief and unhurried dominance, were soon unsettled, burdened by the onslaught – and expectation – which suddenly bore heavily on their slumping shoulders.

When the final whistle blew and the dust settled, Arsenal had had a poor night. They had suffered their first league loss since the opening day and their first defeat on the road in the league since that dismal and pivotal 3-2 loss to a poor Manchester United side in February.

They had also sent a message to their rivals – Arsenal don’t like it up ‘em.

It may be respite on Sunday to face a fading Manchester City side whose garlanded manager holds a deep distaste for tackling – but, until Mustafi returns, we can expect more of the same from every other team Arsenal face.

A crucial period now lies ahead as fans and opposing teams size up just how good and how mentally strong Arsenal are under pressure.

The fact the Gunners failed their first real test with raised expectations does not bode well. Was it a blip or a sign of a weak spirit when the pressure is on? The coming weeks will tell us more.

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